The invention relates to a line driver. A line driver is an electronic buffer amplifier designed to have an output impedance matched to the characteristic impedance of a transmission line. Transmission lines are widely used for conveying electrical signals. To minimize reflections the source and load impedances of the transmission line should be equal to the characteristic impedance of the transmission line. A standard value for video applications is 75 ohms. A buffer amplifier designed to drive a 75 ohm transmission line should have an output impedance of 75 ohms in order to minimize reflections. The buffer sees a load impedance of 75 ohms, i.e. the impedance of the terminated transmission line.
A straightforward approach to implement such a buffer is to provide a low-impedance output stage with a series resistor having a value equal to the characteristic impedance of the transmission line. This approach has several disadvantages. Firstly, the resistance value of the series resistor is subject to tolerances, particularly to process variations if the buffer is integrated on a semiconductor chip, resulting in impedance mismatch. Second, the impedance of the output stage is no longer low and the output impedance of the buffer increases at high frequencies, while the influence of transmission line reflections is most pronounced at these high frequencies. Third, the voltage drop across the series resistance must be regarded as a loss, which is unattractive in low supply voltage applications. This voltage drop is equal to the output voltage of the buffer and is typically 1 V for video signals.
U.S. Pat. No. 5,121,080 discloses a line driver comprising an operational transconductance amplifier (OTA) having a non-inverting and an inverting input and a first and a second current output for supplying currents in response to the voltage difference between the non-inverting and inverting inputs. The first current output is connected to the inverting input, and the second current output, which forms the output of the line driver, is connected to the inverting input through a feedback conductance. An input conductance is connected from the inverting input to ground, and the input signal is connected to the non-inverting input. The currents provided by the first and second current outputs are proportional to each other by a predetermined ratio. By proper selection of this predetermined ratio and of the feedback and input conductances the desired output impedance and overall gain of the line driver into a given load impedance can be achieved. This known line driver constitutes a line driver having a specified output impedance without the voltage amplitude loss of the series resistor approach. The output impedance of this known line driver, however, is still dependent on variations in the selected output current ratio and on the values of the selected input and feedback transconductances.